The invention relates generally to asphalt plants. More specifically, the present invention relates to an asphalt plant which captures and mitigates fugitive emissions and which uses a centralized media burner to supply process heat energy to the various plant components.
On asphalt plants it is desirable to have a variety of air pollution control measures. The asphalt making process, by its very nature of heating and processing the bituminous asphalt components, produces a considerable quantity of undesirable hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), particulate matter and other emissions which constitute the unfortunate signature plume of an asphalt plant, commonly referred to as xe2x80x9cblue smoke.xe2x80x9d In addition to being a source of air pollution, asphalt plants are noisy and visually unappealing, owing to their network of open conveyors, hoppers, bins, blowers and other heating and material handling equipment. Accordingly, asphalt plants in general are regarded as quite a nuisance, especially in and around residential areas.
The typical asphalt plant has high energy requirements. The drum dryer/mixer typically includes a gas burner to dry the aggregate material and to heat the mixing zone to foster adequate mixing of the aggregate with the liquid asphalt. The asphalt material contained in the asphalt storage tanks must be constantly heated to maintain the asphalt cement in its liquid state, and thus another gas burner or similar heating system is required in order to constantly heat the storage tanks. Thus, burner emission are created at both the asphalt storage tanks and at the drum dryer/mixer.
Moreover, the volatile components of the heated asphalt cement as well as the finished asphalt create a certain amount of fugitive emissions as the asphalt components and the finished asphalt are stored, mixed, and transported through the plant. Furthermore, the asphalt cement storage tanks and the asphalt storage silos are usually vented in order to prevent undue pressure build up, especially on hot days, which further complicates the fugitive emission problem. Additional fugitive emissions are created when the finished asphalt material is loaded onto trucks for transport to a job site.
One approach to alleviating the fugitive emission problem has been to enclose portions or all of the plant in order to minimize the amount of leakage from the ductwork and conveyors in the plant. Such an approach, an example of which is described more fully in U.S. Pat. No. 5,620,249, does not provide an improved mitigation system and is typically best suited for applications in which the plant can be made very compact, which is not always feasible.
Attempts have also been made to apply flameless media burner technology to asphalt plants. Media burner technology uses a bed or matrix of ceramic materials which act as a flame arrestor, thereby controlling the rate and temperature of the combustion process. Externally mixed fuel is added to the media burner, which is pre-heated until a self-sustaining combustion is initiated. Ideally, a very efficient centralized media burner should be able to supply heat to the various process components, so that the maximum amount of energy is extracted from the consumed fuel. Unfortunately, existing media burner technology has proven unsatisfactory for asphalt processing plants. The externally mixed fuel components have proven to be too explosive for safe, everyday applications.
Accordingly, there exists a continuing need for an improved asphalt plants.
According to a first aspect of the invention, an asphalt plant includes a plurality of asphalt processing components, with the plurality of processing components including a first set of components producing volatile emission and a second set of components requiring process heat. A central burner assembly is disposed separately from each of the plurality of processing components, with the central burner assembly being adapted to supply heat energy in the form of heated gas to satisfy the process heat requirements of the second component set. A first duct system is in flow communication with the first component set and the central burner assembly, with the first duct system including a fan and being adapted to capture a portion of the volatile emissions produced by the first component set and to convey the captured emissions into the central burner assembly for mitigation. A second duct system is in flow communication with each of the components in the second component set and the central burner assembly, with the second duct system including a fan and being adapted to convey heated gas from the central burner assembly to the second component set.
In further accordance with a preferred embodiment, the first set of components may include an asphalt cement storage tank, an asphalt storage silo, a drum dryer/mixer having a mixing zone, and/or a truck loading area having a substantially sealed enclosure. At least one component of the first set of components may include an enclosure connected to the first duct system.
The central burner assembly may include an air inlet plenum, with the first duct system being connected to the air inlet plenum. Further, the second component set may include a rotary drum dryer/mixer, and the second duct system may include an insulated portion for conveying heat to the drum dryer/mixer. The asphalt plant may include a cement storage tank, with each of the storage tank and the central burner assembly including a heat exchange unit, with the heat exchange units being adapted to scavenge heat from the central burner assembly and convey the heat to the storage tank.
Preferably, the central burner assembly comprises a media burner having an enclosed combustion chamber defined in part by a top wall, a bottom wall, and an interconnecting sidewall. A portion of the combustion chamber preferably contains a matrix of ceramic members. The media burner preferably includes a fuel delivery system, with the internal delivery system being adjustable to permit the fuel to be injected at different locations within the media burner.
Still preferably, the first duct system may be connected to an air plenum for delivering captured fugitive emissions to the media burner, and an air valve may be provided for controlling the flow of air from the air plenum to the combustion chamber. The air valve may include a baffle slidably mounted adjacent an air inlet opening in the bottom wall, with the baffle being moveable between an open position removed from the air inlet opening and a closed position covering the air inlet opening.
In accordance with another aspect of the invention, a central media burner is provided for providing process heat in the form of heated gas to a selected set of asphalt processing components, with the central media burner being separate from each of the processing components. The central media burner comprises an enclosed combustion chamber defined in part by a top wall, a bottom wall, and an interconnecting sidewall. A portion of the combustion chamber contains a matrix of ceramic members, with at least one of the walls defining a gas outlet. The combustion chamber is adapted to supply thermal energy in the form of heated gas. An adjustable internal fuel delivery system delivers fuel to a selected location in the combustion chamber portion, and an air inlet plenum delivers combustion air to the combustion chamber. A duct system communicates the heated gas from the gas outlet to the selected set of processing components, whereby the heated gas is supplied through the duct system to each of the processing components to satisfy the process heat requirements thereof.
In accordance with yet another aspect of the invention, an asphalt plant comprises a plurality of asphalt processing components, with a first set of components producing volatile emissions, and a second set of components requiring process heat energy. A central media burner produces process heat energy in the form of heated gases, with the media burner having an enclosed combustion chamber defined in part by a top wall, a bottom wall, and an interconnecting sidewall, with a portion of the combustion chamber containing a matrix of flame arresting ceramic members. The central media burner, which is separate from each of the plurality of asphalt processing components, also includes an inlet and an outlet. A fuel delivery system is provided to deliver combustion fuel directly to the combustion chamber. A first duct system including a fan is in flow communication with the first component set and the central media burner inlet, and captures a portion of the volatile emissions produced by the first component set and conveys the captured emissions to the inlet of the central media burner for mitigation. A second duct system includes a fan and is in flow communication with the second component set and the central media burner outlet, and conveys heat energy from the outlet of the central media burner to the second component set.
In accordance with a still further aspect of the invention, an asphalt plant comprises a drum dryer/mixer requiring heat energy and producing volatile emissions, a flameless media burner assembly, a first duct system and a second duct system. The media burner assembly is remote from the drum dryer/mixer, with the media burner assembly having an enclosed combustion chamber and a fuel delivery system extending into the combustion chamber. A portion of the combustion chamber houses a matrix of ceramic members, and the fuel delivery system has a fuel port to convey fuel to the combustion chamber portion. The first duct system is in flow communication with the drum dryer/mixer and the media burner assembly, with the first duct system including a fan and being adapted to capture a portion of the volatile emissions produced by the drum dryer/mixer and to convey the captured emissions into the media burner assembly for mitigation. The second duct system is in flow communication with the drum dryer/mixer and the media burner assembly, with the second duct system including a fan and being adapted to convey heat energy in the form of heated gas from the media burner assembly to the drum dryer/mixer.
In accordance with yet another aspect of the invention, an asphalt plant comprises an asphalt processing component that produces volatile emissions and that requires process heat, a flameless burner assembly, and a first and second duct system. The flameless burner assembly supplies heat energy to the processing component, with the flameless burner assembly being separate and spaced apart from the processing component. The first duct system is in flow communication with the processing component and the flameless burner assembly and captures at least a portion of the volatile emissions produced by the processing component and conveys the captured emissions into the flameless burner for mitigation. The second duct system is in communication with the processing component and the flameless burner assembly, and supplies heat energy from the flameless burner assembly to the second component set to thereby substantially satisfy the process heat requirements thereof.
These and other objects, features and advantages of the present invention will become readily apparent to those skilled in the art upon a reading of the following description and claims.